Tiny parasitic worms that cause chronic illness in millions of sub-Saharan Africans may increase their chances of contracting HIV, according to a new report. This study comes just a week after another set of researchers announced a different theory regarding a factor that could increase Africans’ susceptibility to the HIV virus, hypothesizing that a genetic variant found in people of African descent raises the risk of HIV infection.

In the latest study, researchers infected monkeys with the worms that cause schistosomiasis, and then injected them with a form of the HIV virus. They found that much lower amounts of the virus were necessary to give AIDS to the monkeys that had the parasitic worms, as compared to parasite-free monkeys. The phenomenon… needs to be verified in humans. But with primates a generally reliable model of AIDS pathology, it could help explain why sub-Saharan Africa, where 160 million people are infected with schistosomiasis, has 10% of the world’s population and 62% of its AIDS cases [Wired News].

Schistosomiasis, seen primarily in developing countries, is caused by tiny flatworms that live in snail-infested freshwater like rivers and lakes. When people wade, swim or bathe in contaminated water, worms bore through the skin and travel in the blood, causing anemia, diarrhea, internal bleeding, organ damage and death. [Evan Secor of the U.S. Centers for Disease Control] said the parasitic worm infection may undercut the immune system’s ability to fight off HIV infection and may make it easier for HIV to get into white blood cells [Reuters].

The study, which was published in the journal PLoS Neglected Tropical Diseases, might also explain why HIV is spread more easily via heterosexual sex in Africa than in other parts of the world. This could be because the urinary form of schistosomiasis, which affects up to 50 per cent of women in parts of Africa, damages the lining of the vagina, the first defence against HIV [New Scientist].